The PowerBook Duo 250 has an active-matrix display which has 16 grayscales, while the PowerBook Duo 270c has a color active-matrix display with 256 colors at 640 by 480 pixel resolution and thousands of colors at 640 by 400 pixel resolution.
The PowerBook Duo 270c defaults to the 640 by 480 pixel, 256 color mode. By making a selection under Options in the Monitor control panel, then restarting, the Duo 270c can be switched to show 640 by 400 pixels in thousands of colors. When in this mode, a black band will appear at the top and the bottom of the screen.
Since contrast adjustment produces little effect on a color active-matrix display, the Duo 270c does not have a contrast control, while the Duo 250 does.